Storage tube with electron bombardment induced conductivity target



3,509,414 CED April 28, 1970 B. c. EINSTEIN STORAGE TUBE WITH ELFCTRON BOMBARDMENT INDU GONDUCTIVITY TARGE Filed Aug. 2, 1967 INVENTOR 5. C 6/A/57E7/V United States Patent 3,509,414 STORAGE TUBE WITH ELECTRON BOMBARD- MENT INDUCED CONDUCTIVITY TARGET Bernard C. Einstein, Thousand Oaks, Calif., assignor to International Telephone and Telegraph Corporation, New York, N.Y., a corporation of Maryland Filed Aug. 2, 1967, Ser. No. 657,886 Int. Cl. Hfllj 31/28 US. Cl. 315-11 3 Claims ABSTRACT OF THE DISCLOSURE A storage tube for use as an image orthicon or vidicon or the like including an imperforate dielectric layer on only one side of a metal mesh and decreasing in density outwardly from the mesh. The layer has an electron bombardment induced conductivity characteristic. Primary electrons are directed onto the bare metal side of the mesh. A scanning beam of electrons are accelerated by a grid onto the layer on the other side. The mesh casts a shadow so that areas of the layer can be maintained at cathode potential. These areas act as a virtual suppressor grid. Partial or total loss of resolution is thereby prevented.

BACKGROUND OF THE INVENTION This invention relates to storage tubes, and more particularly to a high resolution target for an image orthicoi, vidicon or other television camera-type or storage tu e.

In the past it has been the practice to employ a storage tube having a target exhibiting an electron bombardment induced conductivity (EBIC) characteristic. Such a target is also known as a secondary emission conduction video storage target. In such a prior art arrangement, the target includes a smoke film, potassium chloride or other insulating layer on a solid, thin conductive sheet.

Prior art EBIC targets have suffered from several serious disadvantages. One of these is poor resolution. For example, when primary electrons from a photocathode strike the target at a velocity of several thousand volts, secondary electrons may escape from the EBIC layer to the accelerator grid conventionally used in the path of the scanning beam. When the number of secondaries collected by the accelerator grid is large and continues for an extended period of time, the layer surface potential is driven to the accelerator grid potential. The scanning beam cannot then restore the layer surface to the cathode potential. This condition is described as run away and it results in a partial or total loss of resolution. It also makes it impossible to store scenes having relatively large highlights or brightnesses.

SUMMARY OF THE INVENTION In accordance with the device of the present invention, a storage tube target is provided including a conductive mesh coated on one side with an imperforate EBIC layer. In this case, the outer criss-crossed surface area of the layer in registration with the mesh is effectively shielded or shaded from the high velocity primary electrons which are directed onto the essentially bare metal side of the mesh. The said layer area in registration thus acts as a virtual suppressor grid, and the escape of an excessive number of secondaries to the accelerator grid is prevented.

The target of the present invention thus has the advantage of high resolution, although it has a simple, rugged and inexpensive construction. It also has a wide dynamic range.

The above-described and other advantages of the present invention will be better understood from the following description when considered in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING In the drawing which is to be regarded as merely illustrative,

FIG. 1 is a side elevational view, partly in section, of a storage tube constructed in accordance with the present invention; and

FIG. 2 is a sectional view through a storage tube target shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawing in FIG. 1, a storage tube is indicated at 10 having an evacuated envelope 11. An electron gun 12 is shown having a cathode 13 for producing a scanning beam of electrons 14. Beam 14 is directed onto a target 15 through an accelerator grid or mesh 16. Primary electrons are directed onto the target 15 from a photosensitive coating 17 fixed to the left of envelope 11. Coating 17 is maintained several thousand volts negative with respect to cathode 13 by a source of potential 19.

Accelerator mesh 16 is maintained about 300 volts positive with respect to cathode 13 by a source of potential 18.

As shown in FIG. 2, target 15 incorporates a woven wire mesh 20 onto which an electron bombardment induced conductivity coating 21 is provided. Mesh 20 may be maintained at from 1 to 40 volts positive with respect to cathode 13. Preferably, mesh 20 is maintained to volts positive with respect to cathode 13.

Coating 21 may be a smoke film layer deposited in any conventional manner. Alternatively, coating 21 may be a potassium chloride layer.

Either coating 21 may be deposited upon mesh 20 by any conventional method. It may be evaporated with or without the use of a lacquer film as is conventional.

Tube 10 may be operated either as an image orthicon or as a vidicon. When operated as an image orthicon, the video output will be taken at accelerator mesh 16. When operated as a vidicon, the video output will be taken at target mesh 20.

In the operation of target 15, the mesh 20 shadows the upper surface of coating 21. Thus, the surface of coating 21 directly above mesh 20 in FIG. 2 will not be driven to the potential of accelerator mesh 16 because primary electrons from coating 17 cannot reach the said shadowed surface. Resolution is thus improved.

Mesh 20 may be made of any convenient metal including but not limited to nickel having 1000 lines per inch with a transmission of about percent.

Typically, coating 21 is imperforate and has a thickness indicated at T of about 10 microns.

Mesh 20 may be either a woven Wire mesh or an electroformed metal mesh.

The density of coating 21 should decrease in the direction from mesh 20 to mesh 16 in order to optimize electron absorption, and the conductivity processes.

Although several materials and dimensions have been described herein, as well as certain operating voltages, the invention is of course not limited to those specifically described. Many changes and modifications of the invention will suggest themselves to those skilled in the art. The true scope of the invention is therefore defined only in the appended claims.

What is claimed is:

1. A storage tube comprising: an evacuated envelope; a target supported in said envelope, said target including an electrically conductive mesh having a substantially imperforate coating fixed to one side thereof exhibiting an electron bombardment induced conductivity characteristic, the otherside of said mesh being substantially bare of said coating; first means to direct primary electrons onto said other mesh side; and second means for directing a scanning beam of electrons onto said coating, said mesh acting as a partial mask for primary electrons to keep the surface of said coating in areas thereon shadowed by said mesh at a potential lower than that elsewhere in said coating, said coating having a density decreasing from said one side of said mesh.

2. The invention as defined in claim 1, wherein said mesh is made of nickel wire having 1000 lines per inch with a transmission of about 50 percent, said coating being a layer of potassium chloride having a maximum thickness of about ten microns from the extreme surface of said one mesh side, said second means including an electron gun having a cathode, and accelerator grid, and means for maintaining said grid positive with respect to said cathode, said first means including a photoemissive coating and a power supply for maintaining said coating negative with respect to said cathode.

3. The invention as defined in claim 1, wherein said coating is a smoke film layer.

References Cited 5 UNITED STATES PATENTS 2,683,832 7/1954 Edwards et a1 31365 2,713,648 7/1955 Gardner 31512 2,776,387 1/1957 Pensak 313-65 3,001,098 9/1961 Schneeberger 313103 X 10 3,213,315 10/1965 Lempert 31365 X 3,213,316 10/1965 Goetze et al 313-65 X FOREIGN PATENTS 829,547 3/1960 Great Britain. 15

JAMES W. LAWRENCE, Primary Examiner V. LAFRANCHI, Assistant Examiner U.S. Cl. X.R. 0 313-65 

